Sporadic rioting in predominantly nationalist Catholic neighbourhoods of the British-ruled province on Tuesday night came on the eve of the anniversary of the Battle of the Boyne, a date unionists celebrate with parades which are considered provocative by nationalists, who favour a united Ireland.

About 200 people threw stones, firecrackers and petrol bombs in the Ardoyne area of Belfast after police moved in to prevent them confronting the passing Orange Order parade on the disputed Crumlin Road.

Two cars were set on fire and dozens of rounds of plastic bullets were fired and police said a number of officers were injured.

Most of the 500 or so parades across the province passed peacefully, but police reported rioting in Londonderry, known as Derry by nationalists, Newry and Armagh, as well as the Markets area in central Belfast.

Sectarian strife

A small Orange Order parade passed the Ardoyne estate in near silence with one drummer keeping time after a government commission ordered marchers not to play their traditional drums or flutes on "The Twelth".

A few dozen residents held a silent protest as they passed, while a small group of women sang the Irish national anthem.

But hundreds of others were pinned by police vans and officers in riot gear into an estate a hundred metresfrom the marchers, a move residents said was heavy-handed.

"It's the same thing every year. It's aggravation," Jim, a 47-year-old health worker watching the parade, said. "We're surrounded up here. It's no wonder the kids have so much hatred."

Unionist and nationalist politicians have called for calm in recent days and urged people not to go into the streets to protest against the parades.

"We must not allow the progress that has been made to be thwarted by those who want to drag us back to the past," Peter Robinson, Northern Ireland's first minister, said.

Police said they had also come under attack on Tuesday evening in the mainly Catholic Markets area of Belfast, with rioters throwing bricks and fireworks and setting a car on fire.

A car was hijacked and set alight in Armagh.

'Orange Order' commemoration

Marchers of the Orange Order brotherhood were marking English king William of Orange's victory over his Catholic predecessor, King James, at the Battle of the Boyne in 1690, considered a key date by unionists in securing British rule in Ireland.

Pipe bands and drummers from Scotland joined local groups decked in orange banners and British flags for hundreds of marches across the province.

"It's a celebration, we don't want any trouble," Eddie Whyte, 42, said, as he marched past Belfast City Hall on Tuesday morning. "If they are offended by the British flag, maybe they shouldn't be living in this country."

Three decades of fighting tore Northern Ireland apart during a period known as the "Troubles".

A 1998 peace agreement paved the way for a power-sharing government of loyalists and nationalists.

Violence has subsided, but police say the threat from dissident groups opposed to the peace deal is higher than it has ever been since it was signed.